TT on a 4 cyl could never be as good as a single setup, EXCEPT in a boxer engine. for the same reason that a TT parallel setup was good in the 300zx. there were two banks of cylinders in the V6 so by using a small turbo on each side, they reduced the amount of piping, and thus reduced lag. it is inevitable that people will be twin turboing their WRXs in the near future, because one of the flaws in its design is that there is only one turbo, and furthermore, its placement is nearer one bank of cylinders than the other.